Where I live, there are both mixed and single sex Catholic Schools.
Guess which two are the highest performing ones? That's right, the single sex ones.
There are also both mixed and single sex Independent Schools.
Guess which ones are the highest performing? That's right, the single sex ones.
In the next borough along, there are both mixed and single sex State Schools.
Can you guess where I'm going with this? Yup, the highest performing ones are the single sex ones.
Catholic - well, Faith, Schools, as I've also worked in CofE and colleagues have worked in the small number of other faith schools - attract and admit all abilities and social backgrounds - the determining factor upon admission is the families' having a religious belief and, where oversubscribed, being a practising Catholic/Anglican/Hindu/Muslim/whatever.
To remove children's access to a different school results in, frankly, shit schools in poor areas and outstanding schools in wealthier ones.
This seems to be deliberately taking access to a good school away from 50% of the future students purely because they are female and they don't deserve it as much as boys.
It's no different in my mind to having two girls tie to win a competition and being told they can't both have it because a boy came 19th but he's a boy, so has to have one of the prizes.